library(terra) # for spatial data manipulation
library(mapview) # for spatial data visualization
MOPEX=vect("C:/Users/VSehgal/OneDrive - LSU AgCenter/TAMU/research_tamu/general_data_access/MOPEX Catchments/MOPEX_catchments_431.shp")
# Display the MOPEX catchments
mapview(MOPEX, col.regions="blue", lwd=2, legend=TRUE, layer.name="MOPEX Catchments") Landscape-scale Soil Hydrology Model
Model Description
Landscape-scale Soil Hydrology Model is a 2-layer soil water balance model which works based on the emergent signature of terrestrial water-energy coupling beyond the pedon scale scale. As general schematic of the model with key model variables is given below:
The nature of the soil-vegetation-atmosphere (SVA) interactions evolve with changes in the rootzone soil moisture. This transition of soil hydrologic state from energy-limited to water-limited regimes is represented as a sigmoid curve, with three distinct phases. Abundance of soil moisture in the SVA system can be characterized as wet decoupled, characterized by weakened linkages between water–carbon–energy cycles due to limiting net radiation and atmospheric resistance, with ET losses close to (or at) the potential rate. As the soil begins to dry, soil moisture has higher control over the temporal variability of ET and is characterized by strong land-atmospheric coupling. As soil dries further, soil moisture can not sustain high atmospheric moisture demand, and evapotranspiration again decouples from the rootzone soil moisture.
The connections between soil water balance, noise-induced variability in climatic forces, and positive land-atmospheric feedback lead to dynamic transitions in soil moisture, potentially resulting in a preferred hydrologic states. When one or more state variables are disturbed, or stress (such as atmospheric moisture demand causing evapotranspirative losses) is gradually increased, the SVA system may shift into a new hydrologic regime. Sustained dry-average conditions reinforce dry anomalies (low evaporation results in low regional precipitation and dry soils, leading to even lower evaporation). Conversely, precipitation can alter the soil hydrologic state to wet-average conditions, thereby increasing regional evaporation and precipitation through moisture recycling. If \(\theta_{r}\) represents rootzone soil moisture, then the threshold at which hydrologic tipping of rootzone soil moisture from wet-to-dry-preferential state occurs is represented at \(\theta_{X, r}\), which is called the “tipping” point of soil hydrological regimes.
This relationship between rootzone soil moisture and evaporative fraction (evapotranspiration over potential evapotranspiration) is represented as a sigmoid function given below:
\[ EF=\frac{ET}{PET}=\frac{EF_{m x}}{\left(1+\left|\left(\frac{\theta_{X, r}\left(\theta_r-1\right)}{\left(1-\theta_{X, r}\right) \theta_r}\right)\right|^m\right)} \]
Here, \(EF_{mx}\) represents the maximum evaporative fraction sustainable by the SVA system. The tipping point of soil moisture is represented as \(\theta_{X}\). Land-atmospheric coupling strength is represented by the parameter \(m\). The input vertical flux to the soil below the surface is considered to be derived from the drainage of the surface profile, which is a non-linear function of the field capacity \(\theta_{fc}\), saturated hydraulic conductivity (\(K_{s}\)), porosity (\(\eta\)) and an empirical power coefficient, \(b\). The same equation is adopted for the rootzone to estimate deep drainage.
\[ D=K_S\left(\frac{\left(\max \left(0, \theta-\theta_{f c}\right)\right)}{\eta-\theta_{f c}}\right)^b \]
Surface runoff is estimated as the saturation excess component of the precipitation, based on the hydrologic state of the surface profile. Surface evaporation is considered as a fraction \(Es_{frac}\) of the net evapotranspiration. The depth of the surface and the rootzone profile are fixed and are assumed to be 0-5 and 0-100 cm respectively.
Key Datasets for Model Development
MOPEX Catchments
MOPEX (Model Parameter Estimation Experiment, (Schaake, Cong, and Duan 2006)) catchments are a collection of hydrological catchments across the continental United States used for studying and modeling various aspects of hydrology. These catchments are part of a large dataset that includes daily measurements of evaporation, precipitation, streamflow, and temperature, along with other properties such as drainage area, elevation, and slope. The primary goal of the MOPEX initiative is to improve the understanding and prediction of hydrological processes by providing a comprehensive dataset for model calibration and validation. We will use one of the MOPEX watersheds in Louisiana for the demonstration of the soil water balance model (Duan et al. 2006).